This article contains two code examples to demonstrate Java concurrency. They stand for very typical usage. By understanding them, you will have a better understanding about notify() and wait(). 1. Some background knowledge synchronized keyword is used for exclusive accessing. To make a method synchronized, simply add the synchronized keyword to its declaration. Then no … Read more
There are 4 different types of inner classes in Java. This post illustrates them by using 4 simple examples. 1. Static Nested Classes class Outer { static class Inner { void go() { System.out.println("Inner class reference is: " + this); } } } public class Test { public static void main(String[] args) { Outer.Inner … Read more
Implement Comparable for a TreeSet
In this post, I will show you how to convert a file to a byte array and then convert a byte array to a file. To convert a file to byte array, ByteArrayOutputStream class is used. This class implements an output stream in which the data is written into a byte array. The buffer automatically … Read more
This post shows two different ways to convert an image to a byte array and convert a byte array to an image. First of all, the byte type in Java is an 8-bit signed two’s complement integer. Its range is [-128, 127]. A byte array is just an array of bytes. An image is essentially … Read more
The following code has a compilation errors. It is confusing because you think somewhere to find the problem. import java.util.*; abstract class Animal{ public abstract void checkup(); } class Dog extends Animal{ public void checkup(){ System.out.println("Dog checkup"); } } class Cat extends Animal{ public void checkup(){ System.out.println("Cat checkup"); } } class Bird extends Animal{ public … Read more
In Java, the PriorityQueue class is implemented as a priority heap. Heap is an important data structure in computer science. For a quick overview of heap, here is a very good tutorial. 1. Simple Example The following examples shows the basic operations of PriorityQueue such as offer(), peek(), poll(), and size(). import java.util.Comparator; import java.util.PriorityQueue; … Read more
1. Collection vs Collections First of all, “Collection” and “Collections” are two different concepts. As you will see from the hierarchy diagram below, “Collection” is a root interface in the Collection hierarchy but “Collections” is a class which provide static methods to manipulate on some Collection types. 2. Class hierarchy of Collection The following diagram … Read more
What is the result of the following program? import java.util.Scanner; public class Looking { public static void main(String[] args){ String input = "1 2 a 3 45 6"; Scanner sc = new Scanner(input); int x = 0; do{ x = sc.nextInt(); System.out.print(x + " "); }while(x != 0); } }import java.util.Scanner; public class Looking { … Read more
Simple example to show how to use Date Formatting in Java
Here are a set of diagrams to illustrate Java String’s immutability. 1. Declare a string The following code initializes a string s. String s = "abcd";String s = "abcd"; The variable s stores the reference of a string object as shown below. The arrow can be interpreted as “store reference of”. 2. Assign one string … Read more